With the recent development in the communication technologies, data traffic is rapidly increasing. It is expected that there will be a further increase in the number of users of smart phones, which would result in a further activation of application services using the smart phones and accordingly a further increase in data traffic. Especially, when a new mobile market beyond the inter-user communication, which includes a communication between a human and a machine and a Machine-to-Machine (M2M) communication using intelligence of a machine, is vitalized, it is expected that it will be impossible to cope with the increase in the traffic transmitted to an evolved NodeB (eNB).
Attention is being paid to a Device-to-Device communication (D2D communication) in both the licensed band of the mobile (cellular) communication and the unlicensed band, such as Wireless Local Area Network (WLAN), as a technology that can solve these problems.
It is worth paying attention to the D2D communication in that the D2D communication can increase the traffic accommodating capability of an eNB and reduce the overload of the eNB when it converges with the cell-based mobile communication. By establishing a D2D link between User Equipments (UEs) within the same cell or adjacent cells and allowing, the UEs to directly exchange data without passing through an eNB, it is possible to reduce the two links including the eNB to only one link.
Research in the unlicensed band are intended to recognize requests generated by a human-to-human communication, a human-to-machine communication, and a machine-to-machine communication, prevent unnecessary waste of wireless resources, and provide a service based on a properly determined local traffic. Therefore, the research is focused on a method for efficiently operating a process of broadcasting and receiving a service and information of contents by a plurality of devices.
In order to control a communication between devices, a process of setting synchronization between the devices is required. A device sets synchronization between devices by using time information received through a Global Positioning System (GPS) reception module or from a synchronous eNB. In the case of setting synchronization between devices by using time information received through a GPS reception module or from a synchronous eNB, the device is inevitably required to access the GPS reception module or the synchronous eNB. However, the synchronous eNB may not be supported according to the communication schemes provided by communication service providers, and the device may be thus unable to set synchronization by using the time information received from a synchronous eNB. Further, when a device is located in a GPS shaded area (e.g., an area between high-story buildings, an area in a tunnel, an indoor area of a building), the device is be unable to receive time information from a GPS satellite and thus unable to set the synchronization.
As described above, when a device is unable to readily access a synchronous eNB or GPS reception module, the device is unable to even start a D2D communication.